Despite advances in other techniques, standard X-ray transmission film mammography is by far the most common clinical diagnostic tool for detecting breast cancer. Unfortunately, the spatial resolution of mammograms taken with existing systems is still below that needed to allow visualization of the very small tumors associated with incipient cancer. The fundamental factor limiting the spatial resolution of film mammograms is not the spatial resolution of the film itself, but rather the convertor phosphors which are included in the film cassette to provide the sensitivity needed to allow reasonable films to be made without subjecting patients to unacceptable levels of radiation. We propose to develop a new mammographic film cassette which will greatly improve the spatial resolution and clarity of images taken with existing film radiography systems. This cassette, which will be readily exchangeable with existing units, will contain an advanced convertor phosphor capable of providing the required sensitivity without the present deterioration of the spatial resolution now caused by the existing convertor layers. With this cassette, it will be possible to take greatly improved mammograms with existing equipment and with no increase in patient dose.